1. Field of the Invention
This invention relates to a bar code verifier designed to check if a bar code disposed on a package or some other object typically by printing meets the requirements of a given standard and, more particularly, it relates to a scanner of such a machine for reading the bar code.
2. Background Art
Japanese patent Laid-Open publication No. 5-120462 discloses a scanner for reading a bar code. This known scanner is provided with a viewing window where a dichroic mirror which is arranged so that the operator can ensure that the scanner is properly placed relative to the bar code to be read.
The dichroic mirror may be so designed as to reflect only rays of red light emitted from a red-light emitting device (red-LED) housed within the scanner and allows rays of light having a wavelength shorter than that of red light to pass therethrough. Consequently any external rays of light having a wavelength close to that of red light emitted from the light source (LED) are reflected by the outer surface of the dichroic mirror and not allowed to enter the scanner whereas rays of light having a relatively short wavelength are allowed to enter and pass through the dichroic mirror and reflected by the bar code to pass through the dichroic mirror again before they get to the operator so that the operator can easily view the bar code if the scanner is properly located relative to the bar code.
On the other hand, a sensor device is arranged within the scanner at a position appropriate for receiving the rays of light reflected by the dichroic mirror and, therefore, the rays of light that have a relatively short wavelength and hence enter the scanner from outside would not get to the sensor device. Consequently, the scanner can read the bar code without being adversely affected by external light.
However, the above described conventional scanner functions properly only when the dichroic mirror operates to show an ideal transmissivity curve (a) as illustrated in FIG. 5.
On the other hand, the line (b) illustrated in FIG. 5 represents a normally observably transmissivity curve of a scanner. Therefore, rays of light having a wavelength within the range A are allowed to pass through the dichroic mirror and reflected by the plane of the bar code, say by 50%, before they reenter the dichroic mirror and are received by the sensor device.
Thus, the known scanner is influenced by external light to consequently degrade the contrast of the image to be picked up by the sensor device to a significant extent.
Japanese patent Laid-Open publication No.5-120462 discloses a scanner using a half mirror in place of a dichroic mirror, although it is obvious that such a scanner is also adversely affected by external light.
While such a degradation in contrast due to the presence of a viewing window may not be significant if a scanner of the type under consideration is used for reading bar codes in a retail shop, it provides a problem of impermissible inaccuracy if the scanner is used for the purpose of verifying bar codes. On the other hand, a viewing window is indispensable for a scanner in order for the latter to be accurately placed relative to a bar code. Therefore, one object of the present invention is to provide a scanner having a viewing window that is free from the above problem.